A dynamo-electric machine including, e.g., a generator or a motor is generally constituted by a stator wound with a coil, and a rotor provided with a permanent magnet. In the generator, the rotor is rotatively driven by an engine or the like to allow a rotating magnetic field produced by the permanent magnet to intersect with the coil, thereby generating an electromotive force on a stator side. On the other hand, in the motor, energization of the coil produces a rotating magnetic field, thereby rotatively driving the rotor. The above dynamo-electric machine can obtain high output in spite of its simple structure, and therefore is used as, e.g., a generator for a motorcycle, or a starter-generator that acts both as a starter and a generator.
FIG. 9 is an explanatory view showing a configuration related to a stator and a rotor of a conventional dynamo-electric machine. The dynamo-electric machine of FIG. 9 is a three-phase (U, V, W) type, and has an outer rotor structure in which a rotor 51 is disposed outside a stator 52. The rotor 51 is connected to, e.g., a crankshaft of an engine and is rotatably disposed outside the stator 52. The rotor 51 is provided with a rotor yoke 53 made of a magnetic material. A plurality of permanent magnets 54 is fixed to an inner circumferential surface of the rotor yoke 53 along a circumferential direction thereof. The stator 52 is provided with a stator core 56 having a plurality of salient poles 55 wound with a coil 57.
In the conventional three-phase dynamo-electric machine, the coil 57 of the stator core 56 is continuously wound around every third pole (salient pole 55), as shown in FIG. 9. The every third pole wound with the coil 57 of the same phase faces a magnet having the same polarity as the pole at the same electric angle. In FIG. 9, every third pole, for example, U-phase poles U1 to U4 face N-pole permanent magnets 54 on central axes thereof. Therefore, in the conventional dynamo-electric machine, the permanent magnets 54 and salient poles 55 are arranged at regular intervals, respectively. That is, the permanent magnets 54 are evenly divided into N-pole magnets and S-pole magnets, and the number of poles N is denoted by 2n. The salient poles 55 are also evenly divided, and the number of poles M on a stator side is denoted by 3m in a case of a three-phase type. Here, n is equal to an integral multiple of m. In the case of the dynamo-electric machine shown in FIG. 9, the number of poles N of the permanent magnets 54 is sixteen (n=8), and the number of poles M on the stator side is twelve (m=4; n=2m).
However, when the above conventional dynamo-electric machine is used as a generator for a motorcycle to ensure current generated in an idling rotation range as much as possible, generated current at a middle to high rotation level becomes larger than a consumption current of the machine, resulting in redundant current. Generation of the redundant current correspondingly increases engine friction (engine load) to lead to unfavorable mileage or loss of horsepower. Further, in the generator itself, heat generated in the coil is increased up to close to its heat resistance limit.
In order to cope with the redundant current, a number of turns of the coil is increased, or a thickness of the stator core is increased to obtain larger coil inductance. This suppresses a current generated at a middle to high rotation level, thereby reducing coil temperature. However, it is now impossible to significantly increase a number of turns of the coil in view of available winding space. Further, restrictions on design of a car body make it impossible to increase thickness of the stator core. Thus, under existing circumstance, various types of forced cooling mechanisms are employed to squirt engine oil, form a cooling groove, or the like, thereby reducing heat generated in the coil. However, when the above countermeasures are employed, it becomes impossible to obtain high output, which is demanded due to future load increase, unless a size of the generator itself is increased. It is difficult to increase the size of the generator in a situation where miniaturization of the machine is requested. Thus, there has been a need to develop a dynamo-electric machine capable of increasing current amount at a low rotation level and, at the same time, suppressing current generated at a middle to high rotation level while maintaining a current machine size.